The present invention relates generally to alarm apparatus. More particularly, the present invention relates to apparatus which a person may carry on his or her person for effecting an alarm in case of personal attack or other danger.
The transmission of sound rather than radio frequency signals to a receiver device, as disclosed in U.S. Pat. No. 4,473,821 to Yang et al, the disclosure of which is incorporated herein by reference, has the added advantage of producing of loud noise to attract the attention of people in the vicinity as well as summoning aid by means of the receiver device. A sound wave transmitter also advantageously does not require compliance with various FCC licensing and procedural requirements.
The personal acoustic alarm system of Yang et al includes a miniature portable transmitter which, when activated by a user, produces an audible alarm signal in the audio range for a fixed duration or until the power supply is exhausted to alert persons in the area of the signal transmitter. This audio alarm signal is formed by a plurality of simultaneously generated individual audio frequency sonic signals each of which is supposed to have a different audio frequency. Each of the signals is generated by an oscillator. Receiver units mounted in fixed locations are tuned to receive the audio frequency sonic signals from a specific transmitter or group of transmitters. The receiver unit acts, through a central control unit, to provide some alarm action such as sounding a siren, turning on a strobe light, or dialing a telephone number.
Other patents illustrating the state of the art of personal alarms include U.S. Pat. No. 5,196,825 to Young, U.S. Pat. No. 5,115,224 to Kostusiak et al, U.S. Pat. No. 4,996,517 to Kringen et al, U.S. Pat. No. 4,737,758 to Meiksin et al, U.S. Pat. No. 4,587,516 to Hiraki, and U.S. Pat. No. 3,914,692 to Seaborn, Jr.
The transmitter oscillators for producing such sound waves have conventionally been RC circuits. Commercially available RC circuits at best typically have tolerances of 1 percent on the resistor and 5 percent on the capacitor to provide an overall tolerance between 1 percent and 5 percent. Such a tolerance range requires a broadband receiver. Since the tolerances for each individual frequency may vary and therefore overlap with similar frequency transmissions of other transmitters in the "real world" and since such tolerances are broad enough to encompass various background noises again found in the "real world", the use of a broadband receiver may inadequately distinguish between inputted frequencies from the corresponding transmitter and inputted frequencies generated by one or more other sources.
When faced with personal attack, it is considered desirable not only to attract the attention of other people in the vicinity but also to produce a sound which is irritating to the ears so that an attacker has an incentive for leaving. A sound produced merely by a plurality of frequencies, as in the Yang et al apparatus, may be too regular and therefore too monotonous to have the desired irritating effect.